MAX17497A View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX17497A

AC-DC and DC-DC Peak Current-Mode Converters with Integrated Step-Down Regulator

Maxim Integrated 

MAX17497A/MAX17497B

AC-DC and DC-DC Peak Current-Mode Converters

with Integrated Step-Down Regulator

The MAX17497A flyback/boost converter can be

designed to operate in discontinuous mode or to enter

into continuous-conduction mode at a specific heavy-

load condition for a given DC input voltage. In continu-

ous-conduction mode, the flyback/boost converter needs

slope compensation to avoid subharmonic instability that

occurs naturally over all specified load and line condi-

tions in peak-current-mode-controlled converters operat-

ing at duty cycles greater than 50%. A minimum amount

of slope signal is added to the sensed current signal

even for converters operating below 50% duty cycles

to provide stable, jitter-free operation. The SCOMPF pin

allows the user to program the necessary slope com-

pensation by setting the value of the RSCOMPF resistor

connected from the SCOMPF pin to ground:

RSC= OMPF 0.5 SE kΩ

where the slope (SE) is expressed in millivolts per micro-

second.

Step-Down Overcurrent Protection

The devices’ step-down regulator includes a robust

overcurrent-protection scheme that protects them dur-

ing overload and short-circuit conditions. A runaway

current limit on the high-side switch current at 1A (typ)

protects the device under short-circuit conditions. One

occurrence of the runaway current limit trigger a hiccup

mode to protect the converter by immediately suspend-

ing switching for 32ms. This allows the overload current

to decay, due to power loss in the converter resistances,

and load before soft-start is attempted again.

Error Amplifier, Loop Compensation,

and Power-Stage Design of the

Flyback/Boost Converter

The devices’ flyback/boost converter requires that prop-

er loop compensation be applied to the error-amplifier

output to achieve stable operation. The goal of the com-

pensator design is to achieve the desired closed-loop

bandwidth and sufficient phase margin at the crossover

frequency of the open-loop gain-transfer function of the

converter. The error amplifier included in the devices is a

transconductance amplifier. The compensation network

used to apply the necessary loop compensation is shown

in Figure 9.

The flyback/boost converter can be used to implement

the following converters and operating modes:

• Nonisolated flyback converter in discontinuous-

conduction mode (DCM flyback)

• Nonisolated flyback converter in continuous-conduc-

tion mode (CCM flyback)

• Boost converter in discontinuous-conduction mode

(DCM boost)

• Boost converter in continuous-conduction mode

(CCM boost)

Calculations for loop-compensation values (RZ, CZ, and

CP) for these converter types, and design procedures for

power-stage components, are detailed in the following

sections.

DCM Flyback

Primary Inductance Selection

In a DCM flyback converter, the energy stored in the

primary inductance of the flyback transformer is ideally

delivered entirely to the output. The maximum primary-

inductance value for which the converter remains in

discontinuous mode at all operating conditions can be

calculated as:

L PRIMAX

≤

(VINMIN × DMAX)2 × 0.4

(VOUTF + VD) × IOUTF × fSW

where DMAX is 0.35 for the MAX17497A and 0.7 for the

MAX17497B, VD is the forward-voltage drop of the out-

put rectifier diode on the secondary side, and fSW is the

switching frequency of the power converter. Choose the

primary inductance value to be less than LPRIMAX.

RZ

CP

CZ

COMPF

MAX17497A

MAX17497B

Figure 9. Programming the Output Voltage of the Flyback/Boost

Converter

Maxim Integrated

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